The preparation of salts in the form of porous particles as well as the use thereof as carrier media for detergents has, in itself, been known for a long time. Thus, in EP-OS 221 776 a process is described which comprises drying an aqueous slurry of sodium carbonate together with detergents to form a powder. This process additionally requires the use of crystal builders. Used as crystal builders are polymeric substances, such as polyacrylates, the molecular weight of which can extend up to the order of 250,000. During drying, which preferably takes place according to the spray-drying process, powders are formed which have a comparatively wide particle size distribution spectrum. The opportunity for influencing the granule size is very limited; it is virtually impossible to prepare particles having a size in excess of 300 .mu.m in this manner. Particles of varying particle size will absorb active substances varyingly, so that particles of varying density are formed. In handling, such as transporting or packing, this can lead to separation of mixtures, resulting in inhomogeneities and, e.g., layers of varying concentration within a single package or a concentration which varies from package to package.
In DE-OS 2 642 035 a process is disclosed which comprises blowing silicate having water of crystallization and a stabilizer by evaporating off the water of crystallization. In this way a product is formed with a low bulk density and particles of greatly varying particle size and a very wide pore spectrum with, in part, very large pores not suited to taking up detergents, since these will ooze out again very easily. As during the swelling process the product is very sticky, there must be a layer of stabilizer on the carrier, to prevent sticking. The processing temperatures are relatively high. Also these granulates have a tendency to cause separation of mixture.
In GB Patent Application 2,019,297 the preparation of granulates, more particularly alkali silicate and/or alkali phosphate-containing granulates is described, in which process a mixture of water-containing or water-releasing material is heated in a granulating apparatus to a temperature below its melting point. As the Examples prove, in this process the water content of the material is only reduced by the order of 10%. Also, the granulates have a very wide particle size distribution, so that sieving is recommended and larger granulates have to be fed to a milling process.
Finally, in DE-PS 3 814 274 the preparation of active sodium carbonate which is more or less pulverulent is described. The particles having a granule size of 0.25 to 0.33 mm serve to remove sulphur dioxide from waste gases. To activate the sodium carbonate the water of crystallization is gradually extracted from it, which drying process may be carried out in a fluidized bed. According to the teachings of this patent, porous granulates, which are especially suited to taking up detergents, are not obtained.
Although a whole series of processes for the preparation of porous salt granulates is already known, there is still a need for improved processes by means of which it is possible to prepare such granulates having good or improved properties. It is the object of the invention to provide a process for the preparation of salts in the form of porous granulates that works economically, is easy to be carried out, has no dust formation or only very little, leads to granulates which are homogeneous, show no tendency to separate in either the loaded or the unloaded state, and which are, above all, utilizable as carrier media for liquid washing powder raw materials in detergent compositions of high bulk density. There objectives are attained by the process for the preparation of solid, porous, water-soluble salt granulates according to the present invention.